Method for electrochemical machining

ABSTRACT

The present invention relates to electrochemical machining (ECM) of conductive materials and can be used for manufacturing stamps, mould tools and other workpieces of complex shape at the finishing stage of the machining process. Method for electrochemical machining with an oscillating machining electrode comprises the step of applying rectangular microsecond current pulses synchronized with the moment when a machining electrode and workpiece electrode are moved to a minimum distance towards each other. During the machining process, the amplitude of current pulses is increased and the pulse duration is adjusted in such a manner that the trailing edge of each pulse corresponds to the moment of maximum electrical conductivity of the interelectrode gap, the amplitude being increased until a predetermined roughness of the surface to be machined is achieved. The present invention is aimed at optimizing the finishing stage of the electrochemical machining process by achieving a predetermined surface roughness with a minimum current intensity.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefits from the Russian Application RU 2011101119 filed on Jan. 12, 2011. The content of this application is hereby incorporated by reference and in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to electrochemical machining (ECM) of conductive materials and can be used for manufacturing stamps, mould tools and other workpieces of complex shapes at the finishing machining stage.

A method for electrochemical dimensional machining is known (USSR Inventor's Certificate No. 717847, IPC B23H3/02, 1977), in which the machining is carried out using a pulse power supply with a steep current-voltage characteristic while one of the electrodes is oscillated and voltage pulses are applied during the phase when the electrodes are moved towards each other, wherein the present voltage pulse value is controlled by selecting voltage spikes at the sites where the electrodes are moved towards each other and moved apart from each other, respectively, the voltage spike values being adjusted by changing the electrolyte pressure at the entry of an interelectrode gap.

A method for electrochemical dimensional machining (Russian Patent No. 2038928, IPC B23H3/02, published on 10 Jul. 1995) is known, wherein the machining is carried out using a pulse power supply with a steep current-voltage characteristic while one of the electrodes is oscillated and voltage pulses are applied during the phase when the electrodes are moved towards each other, wherein the present voltage pulse value is controlled by selecting voltage spikes at the sites where the electrodes are moved towards each other and moved apart from each other, respectively, the pulse supply is adjusted with respect to a moment when the electrodes are moved to a minimum distance towards each other by delaying the pulse supply when a voltage spike is present at the site where the electrodes are moved towards each other, the pulse voltage is applied in advance when a voltage spike is present at the site where the electrodes are moved apart from each other, the machining electrode feed rate being increased until the third local voltage extremum is formed in the middle of the pulse and maintained so that the voltage spike does not exceed a voltage value in the middle of the pulse by more than 20%.

The known methods for electrochemical machining are chatacterised in that they employ millisecond pulses during which the interelectrode gap is filled with anode dissolution products such as sludge and a gas-vapor mixture. Furthermore, when the temperature is increased and small interelectrode gaps are used, the process stability is deteriorated. Accordingly, the known methods do not allow increasing machining performance and quality, and improving shaping precision of the machined surface, particularly in case of heat-proof alloys, heat-resistant alloys, rigid alloys and titanium alloys.

Although said methods employ long pulses (with durations of several milliseconds) they do not allow achieving high current intensity and providing low roughness of the surface to be machined.

A method for electrochemical machining (USSR Inventor's certificate No. 891299, IPC B23P1/04, published on 23 Dec. 1981) is known, in which the anode dissolution process is performed by means of rectangular current pulses in a microsecond range. The current pulse duration is set to be at least equal to the time of the capacitance charging of the electric double layer at the anode at the points located at a minimal distance from the cathode, and no longer than the time of the capacitance charging of the electric double layer at the anode at the points located at a distance from the cathode that is equal to a maximum acceptable interelectrode gap value and characterizes an acceptable error of copying the size of the machining electrode.

A shortcoming of the above method is the lack of technique for selecting pulse parameters which ensure a predetermined surface roughness.

A method for electrochemical machining with an optimal pulse duration is known (U.S. Pat. No. 6,723,223, IPC B23H3/00, published on 20 Apr. 2004). According to this method, a plurality of machining voltage pulses with a predetermined optimal duration are applied to a working gap, said optimal duration being determined on the basis of the maximum value of a localization coefficient for a predetermined size of the working gap.

The above method is the closest prior art technique as regards the present invention and is taken as a prototype.

However, this method and the method previously described have low performance due to the fact that they do not provide the parameter optimization of the machining mode so as to achieve a predetermined surface roughness.

BRIEF SUMMARY OF THE INVENTION

The present invention is aimed at optimizing the finishing stage of the electrochemical machining process by achieving a predetermined surface roughness with a minimum current intensity.

The object set above is attained by providing in one aspect of the invention, a method for electrochemical machining with an oscillating machining electrode comprising the step of applying rectangular microsecond current pulses synchronized with an instant when the machining electrode and workpiece electrode are moved to a mininmum distance towards each other, characterized in that the current pulse amplitude is increased during the machining process, and a pulse duration is selected in such a manner that the trailing edge of each pulse corresponds to the moment of maximum electrical conductivity of the interelectrode gap, said amplitude being increased until reaching a required roughness of a surface to be machined.

In another aspect of the invention, an apparatus for electrochemical machining of a workpiece electrode using a machining electrode, is provided, the apparatus comprising

a current pulse generator, the current pulse generator generating rectangular microsecond current pulses synchronized with the moment of maximum approach between the machining electrode and the workpiece electrode, wherein the process is performed within the following parameter range: pulse duration t=5 . . . 500 μs, amplitude current intensity j=200 . . . 10 000 A/cm², and

a pulse and amplitude regulator, the pulse and amplitude regulator adjusting the amplitude and pulse duration so as the trailing edge of each pulse corresponds to the moment of maximum electrical conductivity of an interelectrode gap.

In a further aspect of the invention, an article of manufacture obtained by a process of the first aspect is provided, wherein the article is characterized by low roughness of the machined surface. Examples of articles of manufacture which can be machined using a method according to the invention include stamps, moulds, other workpieces having complex shape, tools, etc.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The details and advantageous effects of the present invention are elucidated by means of the accompanying drawings, in which FIG. 1 shows a function of surface roughness Ra from pulse duration t for various amplitude current intensities j; FIG. 2 shows oscillograms of current intensity J for various voltage values: U=10 . . . 35 V (electrolyte 5.5% NaCl, interelectrode gap size s=30 μm, electrolyte pressure at the interelectrode gap entry P=3 atm); FIG. 3 shows curves of uniform roughness Ra(j,t)=const and a current pulse parameter selection chart: a—function of maximum acceptable pulse duration t from the amplitude current intensity, b—a curve corresponding to a predetermined roughness, M—working point, R—minimum possible roughness.

DETAILED DESCRIPTION OF THE INVENTION

Most alloys machined using electrochemical machining have their surface roughness decreased when current intensity is increased. When electrochemical machining is performed using direct current, current intensity is limited due to the processes of removing of reaction products from the interelectrode gap. Pulsed electrochemical machining allows achieving higher current intensities, and it has been observed that surface roughness also decreases when the pulse duration is increased (see FIG. 1).

It has been also observed that the current intensity oscillogram generally has three local maxima J_(max1), J_(max2), J_(max3). (FIG. 2). Upon reaching the third maximum (t_(max3)), a rapid increase of the interelectrode gap resistance occurs, and then the current stops. Accordingly, the growth of the pulse duration above t_(max3) is limited due to phase blocking, the beginning of which corresponds to the maximum (J_(max)=J(t_(max3))) in the current oscillogram (FIG. 2), and after which a possibility of electric breakdown of the interelectrode gap significantly increases. Therefore, under described conditions machining modes with the pulse duration over t_(max3) are considered non-operable.

Another aspect of the problem of the pulse parameter selection during electrochemical machining consists in the limitations due to the equipment characteristics. For example, the increase of amplitude current is limited by the maximum current value of the generator. For a given current intensity there is a maximum machining area available on a particular machine.

Therefore, the following factors should be taken into account when selecting the machining modes:

-   1. The increase of amplitude current intensity and pulse duration     allows decreasing roughness of a machined surface. -   2. The increase of amplitude current intensity and pulse duration     has energy limitations. -   3. In order to increase the maximum machining area and to decrease     the energy consumption of the process, the amplitude current     intensity can be advantageously decreased.

The projection of surface Rα=ƒ(j,t) on surface (j−t) results in a collection of curves having uniform roughness (FIG. 3). For a selected set of machining mode parameters (see the table) with respect to coordinates j−t, curve t_(max3)=ƒ(j) is defined, each point of which corresponds to a minimum position in the voltage pulse oscillogram (curve α in FIG. 3). The area of disallowed parameters in which the breakdown of the interelectrode gap is possible lies above said curve. Point R, at which said curve is tangential to a curve of uniform roughness, represents minimal possible surface roughness. In the conditions described, said point corresponds to Rα˜0.02 μm with j=850 A/cm² and τ=40 μs.

Curve b corresponds to the predetermined roughness, which should be higher than that at point R. The first intersection of the curve defined by curves b and α, provides point M with coordinates (j*,t*), at which the predetermined surface quality is obtained with the minimum current intensity.

Reference Measure- Range of Parameter symbol ment unit changes 1 Voltage at the inter- U V 5 . . . 150 electrode gap 2 Interelectrode gap size s μm 10 . . . 60  3 Electrolyte pressure P KPa 100 . . . 5 000  at the entry into the interelectrode gap 4 Current pulse duration t μs 10 . . . 500  5 Electrolyte temperature T₀ ° C. 20 6 Oscillation amplitude Δh μm 200  7 Oscillation frequency f Hz 50 8 Pulse amplitude I and 500 . . . 2000  9 Leading/trailing edge τ_(f) μs 0.5 . . . 1   

The method is realized as follows.

Firstly, the machining is performed at a low current intensity, the pulse duration being adjusted in such a manner that it is shorter than the time required to reach the current maximum t₃ for voltage source generators. In case of voltage source generators, the pulse duration is determined on the basis of the minimum in a voltage oscillogram.

After performing a test machining, the roughness of the machined surface is measured. If this roughness is higher than the predetermined value, the current is increased and the previous step is repeated.

If the roughness is lower or equal to the predetermined value, the obtained pulse duration and amplitude current are taken as optimal parameters at the finishing stage of electrochemical machining.

Further, a particular embodiment of the present method is described. The machining was performed in accordance with the machining mode parameters mentioned in the above table.

The predetermined surface roughness is Rα=0.06 μm. This value was obtained at minimum current intensity 600 A/cm² (FIG. 3) and at current pulse duration t=54 μs, which were determined as follows.

1. The selected current intensity was j=100 A/cm². The pulse duration t=5 ms corresponds to the maximum duration provided by the generator. The surface roughness obtained after machining significantly exceeds the predetermined value.

2. The current intensity was increased to j=300 A/cm², and the pulse duration limited due to phase blocking was t=150 μs. The surface roughness obtained in the course machining exceeds the predetermined value.

3. Step 2 was repeated, wherein the current intensity was increased with a pitch of 50 A/cm², and the pulse duration selected was shorter than the time interval necessary for phase blocking, until predetermined roughness Rα=0.06 μm was obtained.

Thus, the present invention allows achieving a predetermined surface roughness at the finishing stage of the electrochemical machining process with a minimum current intensity and, therefore, increasing the machining performance. 

1. A method of electrochemical machining using an oscillating machining electrode, the method comprising a step of applying rectangular microsecond current pulses synchronized with the moment of maximum approach between the said machining electrode and a workpiece electrode, wherein the machining process is performed within the following parameter range: pulse duration t=5 . . . 500 μs, amplitude current intensity j=200 . . . 10 000 A/cm², wherein the amplitude and pulse duration are adjusted so as to make the trailing edge of each pulse corresponding to the moment of maximum electrical conductivity of an interelectrode gap.
 2. The method according to claim 1, characterized in that the maximum electrical conductivity of the interelectrode gap is determined by first increasing the amplitude or duration of current pulses until a precipitous voltage raise associated with a decrease of electrical conductivity of the interelectrode gap is observed, and then decreasing the amplitude or duration of the current pulses until reaching the value, at which the voltage value at the end of each pulse is minimal.
 3. The method according to claim 1, characterized in that maximum electrical conductivity of the interelectrode gap is determined by first increasing the amplitude or duration of voltage pulses until a precipitous current decrease associated with a decrease of electrical conductivity of the interelectrode gap is observed, and then by decreasing the amplitude or duration of the voltage pulses until reaching a value at which the voltage value at the end of each pulse is maximal.
 4. The method according to claim 1, characterized in that a predetermined roughness of the machined surface is achieved during the machining process by increasing the amplitude of current pulses while adjusting the pulse duration in such a manner that the trailing edge of each pulse corresponds to the moment of maximum electrical conductivity of the interelectrode gap, the amplitude being increased until the predetermined roughness of the machined surface is achieved.
 5. An apparatus for electrochemical machining of a workpiece electrode using a machining electrode, the apparatus comprising a current pulse generator, the current pulse generator generating rectangular microsecond current pulses synchronized with the moment of maximum approach between the machining electrode and the workpiece electrode, wherein the process is performed within the following parameter range: pulse duration t=5 . . . 500 μs, amplitude current intensity j=200 . . . 10 000 A/cm², and a pulse and amplitude regulator, the pulse and amplitude regulator adjusting the amplitude and pulse duration so as the trailing edge of each pulse corresponds to the moment of maximum electrical conductivity of an interelectrode gap.
 6. An article of manufacture obtained by a process of claim 1, characterized by low roughness of the machined surface.
 7. The article of manufacture of claim 6, which is a stamp, mould, or other workpiece having complex shape. 